This invention relates generally to high-voltage SCR arrangements and more particularly to a high-voltage SCR circuit that is incorporated in a high-voltage supply for energizing a magnetron of the type commonly employed in a microwave oven.
Microwave ovens have been developed as domestic appliances for heating and cooking foods by exposure to the energy of microwave radiations. According to modern commercial practice, these domestic microwave ovens employ a magnetron, basically an electronic vacuum tube which converts a DC electrical input into an electromagnetic output in the microwave frequency range. Magnetrons of this type generally include a cathode-heater electrode and an anode electrode and exhibit a unidirectional current carrying characteristic. Such a magnetron further requires a DC potential across the electrodes of on the order of 3000 to 5500 volts to bias the tube into conduction for producing the microwave energy.
In the past, high-voltage power supplies for providing this operating potential ordinarily have included a transformer for stepping up conventional household 120-volt AC line power, together with a rectifier and doubler circuit for generating the required level of DC voltage. Generally, a separate source of low-voltage AC potential is supplied to the heater electrode of the magnetron.
Adjustability of the microwave power level is both desirable and a user convenience; and according to one conventional practice, microwave oven power supplies have been provided with a high leakage reactance transformer and a halfwave voltage doubler or villard circuit. The latter circuit comprises a high-voltage capacitor in series with the high-voltage secondary coil of the transformer and a high-voltage rectifier for blocking reverse current to the capacitor. Moreover, various circuits have been employed heretofore as a control in the primary of the transformer for regulating the amount of current applied thereto, thereby affording a degree of regulation over the power being delivered by the secondary and doubler circuit to the magnetron tube and, consequently, a degree of control over the microwave power output. An alternative prior art control arrangement utilizes a capacitor having two selectable values as the series capacitance of the villard circuit or voltage doubler, thereby providing two selectable power levels to the magnetron. Another control arrangement relies on a variable resistor in the current path to the magnetron for adjusting the amount of current supplied thereto. In the former case, only two selectable power levels are available. Moreover, the special dual value capacitor is a relatively expensive device. In the latter case, a limited range of adjustment is available, and considerable power must be dissipated in the resistor. This requires a relatively expensive resistor and one which is capable of consuming a relatively large current. As will be appreciated the consumption of current generates undesirable heat; and this may have a deleterious effect on other circuit components.
A further prior art arrangement for variable control of the microwave output electrically connects a semiconductor triac is joined to a triggering circuit adapted for selectively varying the portion of the AC cycle during which the triac goes into conduction. The triggering circuit is fed by an additional low-voltage tap on the high-voltage secondary of the transformer and includes either an RC phase shifting network and a semiconductor diac in series between the transformer tap and the control terminal of the triac or, alternatively, a multivibrator circuit connected between the tap and the triac control terminal. In the case of a multivibrator, an additional diode is required in series with the triac. This arrangement therefore contemplates a number of added circuit elements and devices as well as a transformer with a supplementary low-voltage tap on the high-voltage secondary, thus adding considerably to the expense and labor required to produce the high-voltage magnetron supply.